The invention manifests itself in a device suitable for the investigation of optical fibers by means of heterodyne-Brillouin-spectroscopy, where light from a pump laser is directed into one end of an optical fiber line under investigation in such a way to excite spontaneous Brillouin backscattering within this optical fiber line, which is superimposed onto a lightwave, which is derived from the same pumplaser, directed to a detector, from which the output signal is fed into a spectrum analyzer.
Specifically, the invention is related to a non-invasive investigation of permanently installed fiber optic transmission lines, to identify the type of fiber in a fiber optic network. Optical fibers can vary by type and quantity of the dopant and by geometrical parameters, which is especially true for fibers from different sources. It is often necessary to find out, which type of optical fiber is installed at a specific part of a fiber optic network.
A pumplaser excites on its way through a fiber optic transmission line a spontaneous Brillouin scattering. The physical mechanism can be described as scattering of pumplaser light at acoustic waves, which are statistically distributed within the optical fiber. The frequency of the preferably backscattered light is downshifted relative to the pumplaser frequency. The backscattered Brillouin light has a spectrum with one or more peaks, which is characteristic for the specific optical fiber. Like a fingerprint, the spectrum allows for an identification of the optical fiber under test.
To measure the spontaneous Brillouin spectrum in an optical fiber, the backscattered light is superimposed to a lightwave with a specific reference frequency. Both light beams are heterodyned at a photodetector. Beat signals between the reference lightwave and the backscattered Brillouin light can then be analyzed. The method is called heterodyne Brillouin spectroscopy.
In a paper entitled "Longitudinal Acoustic Modes and Brillouin Gain Spectra for GeO.sub.2 -doped Core Single-Mode Fibers" from Shibata, Okamoto and Azuma, published in the "Journal of the Optical Society of America", Volume 6, pp 1167-1174 (1989), an experimental device is described to measure the Brillouin gain profile in optical fiber lines. Light from a pumplaser is directed through an optical isolator and a mechanical chopper into one end of the fiber under test. A second, less intense laser is sent into the other end of the optical fiber line as a probe in the opposite direction relative to the pump laser. The frequency of the probe laser is scanned. Its intensity is detected by a photodetector, phase sensitively demodulated with respect to the chopper frequency and displayed as a function of frequency of the probe laser. In this setup the interaction of the pumplaser with the optical fiber medium can be monitored with the probelaser with high sensitivity. Nevertheless, this experiment requires two lasers of good coherence. Both ends of the optical fiber line under test must be accessible. The frequency scale results from a scan of the probelaser and therefore has to be calibrated, which requires further action.
Another device is described in a paper by Tsun, Wada, Sakai, and Yamauchi, entitled "Novel Method Using White Spectral Probe Signals to Measure Brillouin Gain Spectra of Pure Silica Core Fibres", published in "Electronics Letters", Volume 28, pp. 247-249 (1992). Light from a pumplaser is injected from one end into an optical fiber line under test, inducing spontaneous Brillouin backscattering, whereas from the other end a probe lightwave with a white spectral characteristic is sent into the optical fiber line against the pumplaser direction. Both backwards travelling lightbeams, the spontaneous Brillouin backscattering and the white probelight are superimposed at a photodetector by means of a branching device. An electrical spectrum analyzer processes the output of the photodetector. This device requires, as well as the first method described above, access to both ends of the optical fiber under test.
A paper from Tkach, Chraplyvy and Derosier, entitled "Spontaneous Brillouin Scattering for Single-Mode Optical Fibre Characterization" published in "Electronics Letters", Volume 22, pp. 1011-1013 (1986) communicates about a device, where, via an optical isolator and a branching device, light from a pumplaser is directed into an optical fiber under test to induce spontaneous Brillouin backscattering. A part of the pumplight is diverted at the branching device and sent to a mirror, where it is reflected in itself. A part of this backwards travelling pumplight is combined with a part of the spontaneous Brillouin scattering at the branching device and superimposed at the photodetector. The difference frequency spectrum is displayed with a spectrum analyzer.
A paper from Tsun, Wada and Yamauchi, entitled "Wavelength Dependence of Brillouin Frequency Shifts of Optical Fibres in the 1.55 .mu.m Wavelength Region" published in "Electronics Letters", Vol. 27, pp. 1764-1765 (1991) describes a similar device. A part of the light of a pumplaser is branched off by a first branching device and sent through an optical isolator to a second branching device. There it is combined with a part of the pumplaser induced spontaneous Brillouin spectrum from the optical fiber under test, which was travelling backwards through the first branching device, another optical isolator to the second branching device. The heterodyning of the two lightbeams takes place at the photodetector. The output signal is routed to a spectrum analyzer.
The last two papers mentioned describe devices, where a part of the pumplaser light is superimposed with the pumplaser-induced spontaneous Brillouin backscattering at a suitable photodetector. The difference frequency spectrum contains the full information about the specific Brillouin spectrum or Brillouin gain profile. Calibration of the frequency scale is given by processing with an electronic spectrum analyzer. In contrast to the pump/probe devices mentioned before, only one end of the optical fiber under test must be accessible.
The difference frequency spectrum to be measured corresponds to the frequencies of the acoustic phonons, which participate in the Brillouin scattering process. Depending on the pumplaser wavelength, these frequencies are in the 10 to 20 GHz range. Unavoidable frequency drifts of the unreferenced pumplaser degrade the frequency resolution of such devices to some MHz.